A multi-antenna system for performing precoding data using feedback information will hereinafter be described in detail.
FIG. 1 is a block diagram illustrating a multi-antenna system.
In more detail, FIG. 1 shows a structure of a transmission end. The transmission end may be a base station (BS) (also called a Node-B) or a user equipment (UE) (also called a mobile station). In the transmission end, user data to be transmitted from the transmission end (e.g., the base station (BS)) to a reception end is configured in the form of a single stream or multi-data stream, and is then applied to a channel encoder 101. The channel encoder 101 performs a channel encoding. A modulator 102 performs a constellation mapping process on data. The symbolized data is multiplied by a precoding vector, and the multiplied result is transmitted to each antenna 104.
In the meantime, the precoding matrix information is fed back from a reception end. Preferably, the precoding matrix may be selected by feedback information. A controller 105 selects desired users or desired precoding matrixes using feedback information received from the reception end.
A variety of technologies proposed by the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), for example, a Per Antenna Rate Control (PARC), a Per Stream Rate Control (PSRC), and a Per User Unitary Rate Control (PU2RC), can be implemented with the structure of FIG. 1.
The 3GPP LTE has generally introduced a precoding scheme to a closed-loop multi-antenna system. A representative example of the precoding scheme is a PU2RC or a SIC-based Per User and Stream Rate Control (S-PUSRC).
In the case of the PU2RC, a matrix acquired by the number of transmission (Tx) antennas is extended on the basis of Fourier series, such that the extended resultant matrix is used as a unitary matrix for the precoding.
                                          e            m                          (              g              )                                =                                                    1                                  M                                            ⁡                              [                                                      w                                          0                      ⁢                      m                                                              (                      g                      )                                                        ⁢                                                                          ⁢                  …                  ⁢                                                                          ⁢                                      w                                                                  (                                                  M                          -                          1                                                )                                            ⁢                      m                                                              (                      g                      )                                                                      ]                                      T                          ⁢                                  ⁢                              w            nm                          (              g              )                                =                      exp            ⁢                          {                              j                ⁢                                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                    n                                    M                                ⁢                                  (                                      m                    +                                          g                      G                                                        )                                            }                                                          [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, em(g) is a unitary precoding vector, represents the total number of Tx antennas, and ‘G’ represents the total number of groups of a precoding matrix. ‘n’ represents the n-th antenna, and g′ represents the g-th group. The precoding matrix can be specified by the combination of ‘n’ and ‘g’. ‘m’ is indicative of the m-th beam-forming pattern.
The S-PUSRC scheme uses a switching beam-forming vector as a precoding matrix.P=[a1a2a2N]ai=[lejφi . . . ej(N−1)φi]T,φi=kd sin(φi)  [Equation 2]
In Equation 2, ‘N’ represents the number of antennas, αi represents a precoding vector, ‘k’ represents a wavelength, θ represents a steering direction, and ‘d’ represents the distance between neighboring antennas.
A closed-loop multi-antenna system based on a conventional precoding scheme can support any one of a Space Division Multiplexing (SDM) scheme, a Space Division Multiplexing Access (SDMA) scheme, and a beam-forming scheme. If a single user is selected, the SDM scheme is designed to transmit at least one data stream for the single user. If several users (i.e., multi-user) are selected, the SDMA scheme is designed to transmit data to the multi-user via a specific beam. A specific beam is formed by a beam-forming scheme, and data is transmitted via the formed beam. The SDM scheme is called a single-user MIMO scheme. The SDMA scheme is called a multi-user MIMO scheme. Individual characteristics of the above-mentioned three schemes will hereinafter be described in detail.
The single-user MIMO scheme for use in the closed-loop system has different accuracies according to an amount of antenna weight data which is fed back from a reception end (e.g., a user equipment (UE)), such that a MIMO performance is also changed according to the changed accuracy. Particularly, if the number of antennas is at least 4, the size of associated codebook increases, such that the amount of feedback data also increases.
If the multi-user MIMO scheme uses many precoding matrixes to design a codebook, it has difficulty in grouping the multi-user, such that it is difficult for a MIMO system to be implemented.
Finally, according to the beam-forming scheme, the requested interval between antennas of the beam-forming scheme is different from that of the MIMO scheme, such that the beam-forming scheme cannot apply the beam-forming scheme and the MIMO scheme to the same transmission system at the same time.
The single-user MIMO scheme of the closed-loop system has different accuracies according to an amount of antenna weight data which is fed back from the user equipment (UE), such that a MIMO performance is also changed according to the changed accuracy. Particularly, if the number of antennas is at least 4, the size of associated codebook increases, such that the amount of feedback data also increases.
A reception (Rx) performance is greatly affected depending on methods for designing the codebook. Therefore, there is needed a method for reducing an amount of feedback data simultaneously while designing a codebook having a superior performance. A complexity of a receiver should be taken into account when designing a MIMO codebook.
Conventional codebooks have superior performances in a low-correlation channel, but they have less superior performances under a high channel correlation. There is a growing tendency for the above-mentioned codebooks to have different performances according to an antenna structure and the intervals between antennas.
Also, the conventional multi-antenna system has difficulty in designing a systematic codebook having codebook-based adaptability according to a codebook-size extension and a channel status (i.e., a rank).